Pain is an unpleasant sensation which is a primarily protective mechanism that is meant to bring to conscious awareness that tissue damage is occurring or is about to occur. . Pain .
The sensation of pain is accompanied by motivated behavioral responses (such as withdrawal or defense) as well as emotional reactions (such as crying or fear).
COMPONENTS OF PAIN:
Also called Pathologic pain or chronic pain. It includes inflammatory and neuropathic pain.
Slower in onset.
Starts after 1 second of application and increases slowly over many seconds & sometimes minutes.
Greater duration and less localized. It can lead to almost prolonged and unbearable suffering.
It is associated with tissue destruction.
Dull, aching pain, slow pain, throbbing pain, nauseous pain.
It can occur in skin and deeper tissues.
Conducted by C type fibers.
- superficial layers of the skin.
- certain internal tissues, such as the periosteum, the arterial walls, the joint surfaces, and the falx and tentorium in the cranial vault.
- Deeper tissues are only sparsely supplied with pain endings.
In general, fast pain is elicited by the mechanical and thermal types of stimuli, whereas slow pain can be elicited by all three types.
Pain receptors DO NOT ADAPT AT ALL or if they do, very little.
3. Chemical Substances
4. Muscle Spasm
When muscle spasm, the blood supply is decreased leading to decrease in oxygen supply, increased metabolites collecting at the site leading to pain.
The contracted muscle compresses its own blood vessels leading to more ischemia and more pain.
This sets up a vicious cycle or a positive feedback cycle.
Some chemicals that excite the chemical type of pain:
Some chemicals like Substance P and Prostaglandins enhance the sensitivity of pain endings but do not excite them directly.
Other chemicals may get deposited in different tissues of the body and cause pain, such as urates depositing in the synovial membranes of joints leading to gouty arthritis which is a very painful condition.
- From external receptors
- Travel through dorsal roots of spinal cord
- Make up tracts in spinal cord and brainstem
- From thalamus to primary sensory cortex
- Travel through internal capsule
Lateral Spinothalamic Pathway: Pain & temperatureANTEROLATERAL SYSTEM
Because of this double system of pain innervation, a sudden painful stimulus often gives a "double" pain sensation: a fast-sharp (also called First pain) that is transmitted to the brain by the Aδfibers, followed a second or so later by a slow (Second pain) that is transmitted by the C fibers.
- A-delta fibers terminate in lamina I, V and X (Lamina marginalis of the gray matter)
- C fibers terminate in lamina I and II (substantiagelatinosa of the gray matter)
First Order Neuron
Posterior root ganglion (Cell bodies)
Fibers ascend or descend 1-2 spinal cord segments where they are called the Tracts of Lissaeur
On entering the spinal cord, the pain signals take one of the two pathways:
Neospinothalamic pathway --- Paleospinothalamicpathway
Lamina I of dorsal horn Lamina II & III of dorsal horn
(Lamina marginalis) (SubstantiaGelatinosa)
Second order neurons Second order neurons
Decussate immediately through the anterior commissure & then ascend in the anterior & lateral columns of the opposite side of the spinal cord.
as the Spinal Lemniscus(where fibers of Anterior & Lateral Spinothalamic tract ascend together in the lower part of medulla)
Thalamus (VPL nucleus)
Some fibers carrying the slow pain also relay to the Reticular area, Tectal area and Periacquiductal gray region giving rise to SPINOTECTAL and SPINORETICULAR tract
Third Order neurons
(Some fibers carrying the fibers for slow pain also terminate in the hypothalamus)
Anterior Spinothalamic Tract:
Lateral Spinothalamic Tract:
Small diameter, myelinated as well as unmyelinated fibers.
Multiple types of receptors.
Low velocity: 1-15 m/sec
Poor spatial orientation. Low degree of localization.
Transmits the sensations:
Anterior Spinocerebellar Tract
Posterior Spinocerebellar TractThe spinocerebellar tract
Anterior Spinocerebellar Tract
Posterior Spinocerebellar Tract
Carries information mainly from the muscle spindle and the tendon organs of the trunk and lower limb, regarding position and movement of individual limb muscles.
Axons enter the posterior gray column and terminate in the nucleus dorsalis(Clark’s column).
Ascend on the same side to terminate in the cerebellar cortex.
The degree to which a person reacts to pain varies tremendously. This results partly from a capability of the brain itself to suppress input of pain signals to the nervous system by activating a pain control system, called an analgesia system.
It consists of three major components:
The periaqueductal grayand periventricular areas of the mesencephalon and upper pons surround the aqueduct of Sylvius and portions of the third and fourth ventricles. Neurons from these areas send signals to
the raphe magnus nucleus, a thin midline nucleus located in the lower pons and upper medulla, and the nucleus reticularisparagigantocellularis, located laterally in the medulla. From these nuclei, second-order signals are transmitted down the dorsolateral columns in the spinal cord to
a pain inhibitory complex located in the dorsal horns of the spinal cord. At this point, the analgesia signals can block the pain before it is relayed to the brain.
Transmission in nociceptive pathways can be interrupted by actions within the dorsal horn of the spinal cord at the site of sensory afferent transmission.
This gate mechanism is operated by a balance between excitation in large and small peripheral nerve fibers.
An excess of impulses in large nerve fibers results in the closure of the gate and non-production of pain; on the other hand, relative excess of impulses in small nerve fibers opens the gate and produces pain sensation.
Deep and visceral pain can be decreased by applying irritating substances to the skin overlying deep structures. This is the basis of the use of the Counterirritation for the relief of deep and visceral pain.
There are interneurons in the superficial region of the dorsal root ganglion where nociceptive afferents terminate. Opioid receptors (OR) are located on the terminals of nociceptive fibers and on dendrites of dorsal root horn neurons .
Activation of OR results in a decrease in release of glutamate and substance P leading to reduced transmission of from nociceptive afferents.
E.g: Cardiac pain may be referred to the right arm, the abdominal region, or even the back, neck or jaw.
When pain is referred, it is usually to a structure that developed from the same embryonic segment or dermatome as the structure in which the pain originates. E.g, the arm and heart have the same segmental origin.
CONVERGENCE –PROJECTION THEORY: The basis for referred pain may be convergence of somatic and visceral pain fibers on the same second-order neurons in the dorsal horn that project to the thalamus & then to somatosensory cortex.
It is a sensation of pain in response to a normal, painless stimulus.
E.g. painful sensation from a warm shower when the skin is damaged by sunburn.
Damaged nerve fibers undergo sprouting, so fibers from touch receptors synapse on spinal dorsal horn neurons that normally receive only nociceptive input. This explains why painless stimuli can induce pain after injury. The release of substance P and glutamate leads to excessive stimulation of NMDA receptors on spinal neurons, a term called “wind up” that leads to excessive activity of pain transmitting pathway.
Somatosensory Area –I
Somatosensory Area- II
The term somesthetic cortex is reserved for Somatosensory Area-I only.Somatosensory cortex
Somatosensory cortex- II
Its function is not well known.
It seems to be concerned more with the spinothalamic tract and pain sensibility.
It further elaborates the sensory input received by Somatosensory Area I.
Somatosensory Association Area:
These are Brodmann’s area 5 & 7.
It receives information from area I & II and also from the visual cortex and the auditory cortex and thalamus.
This area is responsible for decoding the sensory information.
Removal of this area causes amorphosynthesis (loss of appreciation of form of objects).
It is defined as pain felt by an amputee that seems to be located in the missing limb.
The theory explaining it is that brain can recognize if sensory input is cut off.
The area of the brain that once received input from the leg and foot now responds to stimulation of the stump, in exactly the same way it did when the limb was attached.
Mirror box therapy is used for treatment of Phantom limb pain.